1. Field of the Invention
This invention relates to a numerical control method and, more particularly, to a numerical control method wherein operation is simple at resumption of machining.
2. Description of the Related Art
If, during the machining of a workpiece by a machine tool controlled by a numerical control unit, a tool should break or a power failure occur and result in suspension of the machining operation, the conventional practice is to take the following countermeasures and then resume machining:
(1) The numerical control unit stores, in advance, a machining resumption starting point. For example, the starting point is set in a digital switch or stored in a non-volatile memory. PA1 (2) To resume machining, the operator manipulates an NC operator's panel to restore a table or tool to the machine origin. PA1 (3) After restoration to the origin is performed, the operator positions the table or tool at the machining starting point by using machining starting point information stored in the non-volatile memory or set on the digital switch. PA1 (4) After positioning at the machining starting point is performed, the operator returns numerical control command data to the beginning. PA1 (5) The operator then causes the numerical control unit to search a program from the beginning of the numerical control command data to the position at which machining was suspended. At this time, M-, S- and T-function instructions are not delivered to a programmable controller unit (PC) and these functions are not performed on the PC or machine side. PA1 (6) After (5) is carried out, the operator observes the M-, S- and T-function data that prevailed at the suspension of machining, which data are displayed on a display of the numerical control unit, manually sets the observed data in the PC and resumes machining.
Thus, at the resumption of machining, the conventional numerical control unit is caused to search the program from the beginning of the numerical control command data to the position at which machining was suspended. In doing so, however, the numerical control unit does not perform a distribution operation. Since arithmetic functions are suspended on the PC side, the PC does not have knowledge concerning the M-, S- and T-function data that prevailed at suspension of machining. Consequently, when the program arrives at the state for resumption of machining, the states of the M-, S- and T-functions are displayed on the display unit and these are set in the PC while being verified visually by the operator. Thus, in resuming machining, operator intervention is a necessity in the prior art. This not only results in a troublesome machining restart operation, but is also inconvenient in that unexpected accidents occur when the operator mistakes the states of the M-, S- and T-functions and erroneously enters them in the PC.